Agent skill

142-java-functional-programming

Use when you need to apply functional programming principles in Java — including writing immutable objects and Records, pure functions, functional interfaces, lambda expressions, Stream API pipelines, Optional for null safety, function composition, higher-order functions, pattern matching for instanceof and switch, sealed classes/interfaces for controlled hierarchies, Stream Gatherers for custom operations, currying/partial application, effect boundary separation, and concurrent-safe functional patterns. Part of the skills-for-java project

View SKILL.md on GitHub Repository
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Install this agent skill to your Project

npx add-skill https://github.com/jabrena/cursor-rules-java/tree/main/skills/142-java-functional-programming

Metadata

Additional technical details for this skill

author
Juan Antonio Breña Moral
version
0.14.0

SKILL.md

Java Functional Programming rules

Identify and apply functional programming principles in Java to improve immutability, expressiveness, and maintainability.

What is covered in this Skill?

  • Immutable objects and Records (JEP 395)
  • Pure functions free of side effects
  • Functional interfaces: Function, Predicate, Consumer, Supplier, custom @FunctionalInterface
  • Lambda expressions and method references
  • Stream API: filter/map/reduce pipelines, parallel streams, toUnmodifiable* collectors
  • Optional idiomatic usage: map/flatMap/filter/orElse* over isPresent()+get()
  • Function composition: andThen/compose
  • Higher-order functions: memoization, currying, partial application
  • Pattern Matching for instanceof and switch (Java 21)
  • Sealed classes and interfaces (Java 17) for exhaustive domain hierarchies
  • Switch Expressions (Java 14), Stream Gatherers (JEP 461)
  • Effect-boundary separation: side effects at edges, pure core logic
  • Immutable collections: List.of(), Collectors.toUnmodifiableList()

Scope: The reference is organized by examples (good/bad code patterns) for each core area. Apply recommendations based on applicable examples.

Constraints

Before applying any functional programming changes, ensure the project compiles. If compilation fails, stop immediately — do not proceed until the project compiles successfully. Verify that maven-compiler-plugin source/target supports the Java features being used.

  • MANDATORY: Run ./mvnw compile or mvn compile before applying any changes
  • SAFETY: If compilation fails, stop immediately — do not proceed until the project compiles successfully
  • VERIFY: Verify maven-compiler-plugin source/target supports the Java features being used
  • VERIFY: Run ./mvnw clean verify or mvn clean verify after applying improvements
  • BEFORE APPLYING: Read the reference for detailed good/bad examples, constraints, and safeguards for each functional programming pattern

When to use this skill

  • Improve the code with Functional Programming
  • Apply Functional Programming
  • Refactor the code with Functional Programming

Reference

For detailed guidance, examples, and constraints, see references/142-java-functional-programming.md.

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